1. Technical Field
The present invention relates, in general, to a method and system, to be utilized with wireless communications systems, having cellular architectures which utilize digital clocked systems (such as Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), or similar technologies), and which interface with public switched telephone networks (PSTNs). In particular, the present invention relates to a method and system, to be utilized with wireless communications systems, having cellular architectures which utilize digital clocked systems (such as TDMA, CDMA, or similar technologies), and which interface with PSTNs, wherein the method and system increase the reliability of such wireless communications systems by avoiding communication failures at the wireless communication system-PSTN system interfaces.
2. Description of the Related Art
The present invention is related to wireless communication systems, and, in particular, to wireless communications systems which have both a cellular architecture (e.g., cellular telephony, personal communications systems) and which utilize CDMA (or similar technologies) and which interface with public switched telephone networks (PSTNs). Wireless communication refers to the fact that transmission between sending and receiving stations occurs via electromagnetic radiation (e.g., microwave) not guided by any hard physical path. Cellular architecture refers to the fact that the wireless system effects service over an area by utilizing a system that can (ideally) be pictographically represented as a cellular grid. CDMA stands for Code Division Multiple Access, which is a type of spread spectrum technology, originally developed for military application and thereafter adapted for civilian use.
Wireless cellular communication utilizing CDMA is the latest incarnation of a technology that was originally known as mobile telephone systems. Early mobile telephone system architecture was structured similar to television broadcasting. That is, one very powerful transmitter located at the highest spot in an area would broadcast in a very large radius. If a user were in the usable radius, then that user could broadcast to the base station and communicate by radio telephone to the base station. However, such systems proved to be very expensive for the users and not very profitable to the communication companies supplying such services. The primary limiting factor, or problem, of the original mobile telephone systems was that the number of channels available for use was limited due to severe channel-to-channel interference within the area served by the powerful transmitter.
This problem was solved by the invention of the wireless cellular architecture concept. The wireless cellular architecture concept utilizes geographical subunits called xe2x80x9ccellsxe2x80x9d and encompasses what are known as the xe2x80x9cfrequency reusexe2x80x9d and xe2x80x9chandoffxe2x80x9d concepts. A cell is the basic geographic unit of a cellular system. Cells are defined by base stations (a base station consists of hardware located at the defining location of a cell and includes power sources, interface equipment, radio frequency transmitters and receivers, and antenna systems) transmitting over small geographic areas that are represented (ideally) as hexagons. The term xe2x80x9ccellularxe2x80x9d comes from the honeycomb shape of the areas into which a coverage region, served via two or more base stations, is divided when the mathematically ideal hexagonal shape is used to represent the usable geographic area of each of the two or more base stations. It is to be understood that, although the mathematically ideal shape of the cell is a hexagon, in practicality each cell size varies dependent upon the landscape (e.g., a base station transmitting on a flat plane will closely approximate the ideal hexagon, whereas a base station transmitting in a valley surrounded by hills will not closely approximate a hexagon due to the interference from the surrounding hills).
The first large-scale wireless communications system utilizing cellular architecture in North America was the Advanced Mobile Phone Service (AMPS) which was released in 1983. With the introduction of AMPS, user demand for bandwidth was initially low until users became acquainted with the power of the system. However, once users became acquainted with the power of cellular, the demand for the service increased. Very quickly, even the extended number of channels available utilizing the cellular concepts of reduced power output and frequency reuse were quickly consumed by user demand in certain geographic areas, and a problem arose with respect to capacity.
Engineers responded to the problem by devising the Narrowband Analog Mobile Phone Service (NAMPS). NAMPS utilizes frequency division multiplexing to transmit three transmit/receive channels in the same bandwidth wherein AMPS had previously only transmitted one transmit/receive channel. Thus, NAMPS essentially tripled the capacity of AMPS. However, even though NAMPS essentially tripled the capacity of AMPS, the extended number of channels available with NAMPS were quickly consumed by user demand in certain geographic areas, and a problem again arose with respect to capacity.
Engineers responded to this new problem by devising Digital AMPS (or DAMPS, also known as TDMA). In DAMPS/TDMA time division multiple access techniques are utilized to multiplex user data together. Furthermore, digital data compression techniques are utilized at both the transmission and reception ends. These techniques give rise to increased capacity, and clarity, even exceeding that of NAMPS. However, as was the case with both AMPS and NAMPS, the increased bandwidth capacity of DAMPS/TDMA has been quickly consumed by user demand in certain geographic areas.
Subsequent attempts to increase cellular telephony bandwidth capacity tended to be variations on the foregoing described themes. However, it became apparent that some new communications technology would be necessary to give rise to any significant increase in bandwidth beyond that available with the foregoing described technologies. It was decided within the industry that such new technology would be standard CDMA, which stands for Code Division Multiple Access.
Notice that in all the foregoing described technologies, the method of using multiple transmit/receive channels with each such transmit/receive channel utilizing a different pair of frequencies was maintained throughout. Standard CDMA breaks completely with this method of communication.
Standard CDMA utilizes cellular architecture and a type of hand-off. However, in standard CDMA, transmission and reception is done by all users on the same frequency. Standard CDMA is able to achieve this feat by insuring that the signals from different users are adjusted such that the signals do not interfere with each other to the point of being unable to understand the messages from the different users.
The way in which standard CDMA works is somewhat analogous to a situation in which two English speaking persons are communicating in a room wherein many other non-English speakers are also communicating in a language which the two English speakers do not understand. Since the two English speakers do not understand the language spoken by the non-English speakers in the room, the conversations of their non-English-speaking counterparts will be interpreted by the two English speakers as meaningless xe2x80x9cnoise.xe2x80x9d Consequently, since the English speakers will attach no meaning to the xe2x80x9cnoise,xe2x80x9d the English speakers will be able to disregard the xe2x80x9cnoisexe2x80x9d and continue to engage in their conversation provided that they both speak loudly enough so that each can be understood by the other despite the xe2x80x9cnoisexe2x80x9d generated by their non-English-speaking counterparts. This is true even though all persons in the room are talking, or communicating, in the same band of sound frequencies which the human ear can hear.
Standard CDMA is able to achieve the same affect by modulating the signal of each user within a particular cell with a xe2x80x9cpseudo-noisexe2x80x9d code which, in effect, will make each user in the cell appear as if each user were, in effect, xe2x80x9cspeaking a different language,xe2x80x9d thereby insuring that the meaning of a signal generated by one user within the cell will not be drowned out by the meaning contained within the signal generated by one or more other users in the cell. Provided, of course, that each user speaks xe2x80x9cloudlyxe2x80x9d enough (or transmits enough power) to be understood over the xe2x80x9cnoisexe2x80x9d generated by the other users in the CDMA cell.
Standard CDMA utilizes digital data technology to achieve the foregoing. Standard CDMA utilizes complex digital codes to modulate user data prior to transmission within a cell. The standard CDMA pseudo-noise codes are chosen such that a modulated signal, when transmitted upon a carrier frequency within the cell, approximates white (or Gaussian) noise, and does not greatly interfere with any other signal transmitted upon the same carrier frequency within the cell. Upon reception, a similar pseudo-noise code is used to demodulate the signal and recover the data that was transmitted.
When digital data technology is utilized with the standard CDMA pseudo-noise codes, it is necessary for all transmitters and receivers within a cell to be synchronized to the same digital clock. This synchronization is provided by use of a xe2x80x9cpilotxe2x80x9d signal which is transmitted by the base station. Each mobile subscriber unit within a cell xe2x80x9clocksxe2x80x9d to this pilot signal and thereafter utilizes it as the clock signal for digital data-processing.
In standard CDMA, each base station transmits and receives on the same carrier frequency. Furthermore, in standard CDMA, each base station transmits the same period digital code which is utilized as the pilot signal within each cell. Ordinarily, such a situation would give rise to severe interference between cells. Standard CDMA avoids this problem by phase-shifting (or time-staggering) the pilot signal, or digital code, transmitted within adjacent cells. Within standard CDMA, the carrier signal, pilot code, pseudo-noise codes, and phase-shifting (or time-staggering) of the pilot codes utilized in adjacent cells have all been chosen to work together such that inter-cell interference is minimized. Thus, not only does standard CDMA ensure that users in each cell appear to each other as if they are xe2x80x9cspeaking different languages,xe2x80x9d but standard CDMA ensures that adjacent cells appear to each other xe2x80x9cas ifxe2x80x9d each cell was in fact xe2x80x9cspeaking a different language.xe2x80x9d
It has been stated that when digital data technology is utilized with the standard CDMA pseudo-noise codes, it is necessary for all transmitters and receivers within a cell to be synchronized to the same digital clock. This synchronization is provided by use of a xe2x80x9cpilotxe2x80x9d signal which is transmitted by the base station. Each mobile subscriber unit within a cell xe2x80x9clocksxe2x80x9d to this pilot signal and thereafter utilizes it as the clock signal for digital data-processing. The question naturally arises as to the origin of the clock signal used by the CDMA system.
The answer is that the clock signal originates with the Global Positioning System (GPS). The GPS is a network of geostationary satellites which is utilized to provide precise global positioning. Each GPS satellite contains a clock synchronized to the clocks on the other GPS satellites. One of the features of the GPS is that it emits a xe2x80x9cping,xe2x80x9d or clock signal, every 20 msec. Because each GPS satellite is geostationary, each GPS satellite is at roughly the same distance from the earth""s surface (i.e. Geostationary Height). Consequently, each xe2x80x9cpingxe2x80x9d from a GPS satellite reaches the earth""s surface essentially simultaneously.
Because each xe2x80x9cpingxe2x80x9d reaches the earth""s surface essentially simultaneously, CDMA utilizes such pings as its system clock. Thus, the GPS 20 msec ping provides an effective xe2x80x9cclockxe2x80x9d to synchronize the CDMA transmitters and receivers, and is consequently utilized for that purpose. Thus, the GPS provides an effective way to synchronize a CDMA network which may be spread over a large geographic area.
In many rapidly developing, but previously undeveloped, areas of the world, such as the former Soviet Union, and the Central and South American republics, only CDMA systems are in place. That is, no substantial pre-existing PSTNs are in place. However, in long-developed areas of the world, such as the United States of America, Canada, and the European Union, there are extensive infrastructures of PSTNs present. In such areas, it is necessary for CDMA systems to interface with the PSTN systems in order for CDMA to be commercially viable and to provide seamless communications services to the residents of such areas. Such interfacing poses multiple problems, but one of the most significant arises from the fact that the timing signals utilized by the CDMA systems and the PSTN systems are not synchronized.
A PSTN is a common carrier network that provides circuit switching for the general public. It is usually a domestic communications network that is accessed by telephones, private branch exchange trunks, and data equipment such as modems. One common type of data carried by PSTNs is digitized voice data.
The human voice amounts to an analog (continuous time) signal. However, from a data communications standpoint, it has been found that transmission of the human voice in digital (discrete time) format produces more acceptable results. Consequently, it is necessary to convert the human voice, which is an analog signal, to a digital signal. After transmission, the digital signal is a re-converted to an analog signal which the human ear can hear.
It has been found empirically, that a human voice signal containing at least frequencies up to the 4000 hertz range is acceptable to most listeners. Consequently, it is necessary to sample the voice signal at two times that frequency such that frequencies up to the 4000 hertz range can be adequately captured. That is, it has been found that sampling a voice signal 8,000 times a second will result in acceptable performance.
One way in which the analog to digital conversion is done is known as Pulse Code Modulation (PCM). In PCM systems the analog signal is sampled once every 8000 seconds, which equates to 1 PCM sample every 125 micro-seconds. When a sample occurs, the magnitude of the analog voice signal is noted. Thereafter, some relative scale is utilized to denote that magnitude. Normally, three bits (binary information units, typically denoted by the symbols xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d) are utilized to quantize the analog signal digitally.
Since a PCM system samples data at specific time intervals, a clock signal is needed to synchronize the system. In a PSTN, such a clock signal is derived from what is known as the xe2x80x9cPSTN Clock.xe2x80x9d The PSTN Clock is derived from a centrally located atomic clock located at some central geographic location. There are various of these PSTN Clocks scattered throughout the world. However, for the purposes of this discussion, the central fact to be gleaned is that such PSTN Clocks are not synchronized with the GPS clocks utilized to synchronize the CDMA systems. This lack of synchronization can give rise to several problems, one of which is illustrated in FIG. 1.
Refer now to FIG. 1. FIG. 1 is a partially schematic diagram which will be used to illustrate problems that arise due to the fact that the clocks used to control CDMA systems and PSTN systems are not synchronized. Shown in FIG. 1 is CDMA voice coding subsystem 100. On the right-hand side of FIG. 1 appears PSTN system 102. PSTN system 102 is utilizing PCM and is delivering a PCM input stream 104 to CDMA voice coding subsystem 100. Further shown is that PSTN system 102 utilizes PSTN clock 106, which as has been discussed, is some type of atomic clock at some defined ground-based location.
Shown is that within CDMA voice coding system 100 resides a digital signal processor (DSP) 110. Contained within DSP 110 is PCM-CDMA encoder 112 which accepts PCM sample blocks, signal processes (or encodes) them, and delivers such encoded blocks to CDMA system 108 which appears on the left-hand side of FIG. 1.
Upon receipt of each PCM sample, PCM sample detection circuitry (not shown) interrupts DSP 100 in order to inform DSP 100 that a PCM sample has been received on the PSTN input stream 104. In response to this interrupt, DSP 100 keeps a count of the number of PCM samples received during a particular time interval; furthermore, DSP 100 loads the received PCM sample into a PCM sample input buffer (not shown).
Shown is that CDMA system 108 is controlled, or synchronized by, GPS clock 114. Consequently, when the 20 msec GPS xe2x80x9cpingxe2x80x9d occurs, CDMA system 108 alerts DSP 110 to the fact that the 20 msec ping has occurred. In response, PCM-CDMA encoder 112 retrieves the stored PCM samples from the PCM sample input buffer (not shown), effectively emptying the PCM sample input buffer (not shown) wherein the previously received PCM samples had been stored. After retrieval, PCM-CDMA encoder 112 processes the retrieved PCM sample block and creates a CDMA packet and places the created CDMA packet into a CDMA packet output buffer (not shown). Thereafter, the created CDMA packet is transmitted from CDMA voice coding subsystem 100 under the dictates of GPS clock 114. The CDMA packet leaves CDMA voice coding subsystem 100 via CDMA packet output stream 116.
An essentially reciprocal operation occurs in the reverse direction. That is, CDMA packets enter CDMA voice coding subsystem 100 via CDMA packet input stream 118. Upon receipt of each CDMA packet, CDMA packet detection circuitry (not shown) interrupts DSP 100 in order to inform DSP 100 that a CDMA packet has been received on the CDMA packet input stream 118. In response to this interrupt, DSP 100 places the received CDMA packet into a CDMA packet input buffer (not shown) and directs CDMA packet-PCM sample decoder 120, upon completion of any processing it may be engaged in, to thereafter accept the received CDMA packet, decode it into PCM samples, and place the PCM samples into a PCM sample output buffer (not shown). Thereafter, the PCM samples are read out of the PCM sample output buffer under the dictates of the PSTN clock 106.
Notice that, irrespective of the direction of flow through CDMA voice coding system 100, since PSTN clock 106 and GPS clock 114 are not exactly synchronized (because the clocks do not communicate), some potential data loss is likely. It has been noted that GPS clock 114 produces a ping every 20 msec. It is also been noted that the PCM system utilizes PSTN clock 106 pulses to produce a PCM sample every 125 micro-seconds (e.g., 1 sec/8,000 samples). Consequently, if PSTN clock 106 and GPS clock 114 were perfectly synchronized (i.e., 20 msec measured on GPS clock 114 was exactly the same as 20 msec measured on PSTN clock 106, and the transition edges of the clocks occurred precisely the same instances), there would be 160 PCM samples clocked through CDMA voice coding subsystem 100, on both PCM input stream 104 and PCM output stream 122, respectively, every 20 milliseconds.
Unfortunately, for the reasons discussed above, PSTN clock 106 and GPS clock 114 are not synchronized. That is, during the normal course of operation of the systems the transition edges of the clock do not occur at the same time or at the same rate (i.e., 20 msec as measured by GPS clock 114 will tend to be slightly different that 20 msec as measured by PSTN clock 106). Furthermore, in the event that the clocks differ by more than 1 PCM sample interval (i.e., by more than 125 micro-seconds) sample transmission will eventually begin to trail behind that necessary and eventually data will be dropped due to the finite size of the buffers. This reality can be made clear by a simple example related to PCM input stream 104.
Assume that the 20 msec ping of GPS clock 114 is either xe2x80x9claggingxe2x80x9d or xe2x80x9cleadingxe2x80x9d PSTN clock 106 by a 250 micro-seconds. That is, for every 20 msec deemed to have elapsed by GPS clock 114, according to PSTN clock 106 the elapsed time appears to be 20 msec plus/minus 250 micro-seconds. Admittedly, from the standpoint of a 20 msec interval, plus/minus 250 micro-seconds does not seem that significant, since such lagging or leading amounts to only 1.25% of the 20 msec period.
However, when viewed from the standpoint of the buffers (not shown) of CDMA voice coding subsystem 100, it can be seen that the such leading or lagging can become very significant. If GPS clock 114 is lagging PSTN clock 106 by 250 micro-seconds, then when GPS clock 114 pings, 162 PCM samples will have been collected from PCM input stream 104, rather than PCM samples. Consequently, when PCM-CDMA packet encoder 112 removes 160 PCM samples from the PCM sample input buffer (not shown), two residual PCM samples will remain in the PCM sample input buffer (not shown).
Assuming that GPS clock 114 and PSTN clock 116 remain unsynchronized it can be seen that the PCM packet input buffer (not shown), which has finite capacity, will eventually become full and consequently data will be lost.
If GPS clock 114 is leading PSTN clock 106 by 250 micro-seconds, then when GPS clock 114 pings, 158 PCM samples will have been collected from PCM input stream 104, rather than 106 PCM samples. Consequently, when PCM-CDMA packet encoder 112 removes the PCM samples from the PCM sample input buffer (not shown), it will find that only 158 PCM samples are present and consequently will be unable to construct the appropriately sized CDMA packet.
An analogous state of affairs exists with respect to CDMA packet input buffers (not shown) and the PCM output, or transmit, buffers (not shown). That is, if GPS clock 114 is lagging PSTN clock 106 by 250 micro-seconds, then the when the GPS clock 114 pings, two PCM sample intervals will have transpired with no PCM samples being ejected on the PCM output stream 122. If this state of affairs continues, there will be noticeable xe2x80x9cdata dropxe2x80x9d at relatively periodic intervals, which has been empirically determined to provide unacceptable service to users. That is, a human user can hear and be conscious of such xe2x80x9cdata dropsxe2x80x9d and finds such occurrences rankling. Conversely, if GPS clock 114 is leading PSTN clock 106 by 250 micro-seconds, when GPS clock 114 pings, there will still be to PCM samples in the PCM sample output buffer (not shown). Consequently, if this state of affairs continues, the PCM sample output buffer (not shown) will eventually fill and data will be lost.
The foregoing problems associated with the potential CDMA clock and PSTN clock mismatching have been recognized in the prior art. With respect to the PCM sample input buffer problem noted above, the solution that has been effected under the prior art has been to constantly interrupt DSP 110 upon every receipt of a PCM input sample on PCM input stream 104. These interrupts allow DSP 110 to keep a running count of the number of PCM samples in the PCM sample input buffer. Consequently, when GPS clock 114 pings, DSP 110 can determine if more or less PCM samples are present in the PCM sample input buffer then there should be. In response to such determination, DSP 110 either discards the excessive samples present (e.g., when the samples in the PCM sample input buffer are greater than 160 in number), or duplicates the last PCM sample in the PCM sample input buffer when an inadequate number of PCM samples is present (e.g., when the samples in the PCM sample input buffer are less than 160 in number).
An analogous solution has been applied to the problems associated with the CDMA packet input buffers and PCM sample output buffers discussed above. That is, DSP 110 is interrupted every time a PCM sample is clocked out of the PCM sample output buffer. Consequently, DSP 110 is able to keep count of the number of PCM samples in the PCM sample output buffer and is able to discard PCM samples or add PCM samples to the PCM sample output buffer as appropriate in order to ensure that no CDMA input packets are dropped such that no data outage is experienced by users of PSTN system 102. That is, DSP 110, by using a count kept based on the multiple interrupts, is able to control the PCM sample output buffer such that data drop is not detectable by a human user and such that the CDMA packet input buffer does not overflow.
While the foregoing described solutions to the problems associated with lack of synchronization between CDMA system clocks and PSTN system clocks works well, it is also apparent that the system generates a tremendous number of interrupts to DSP 110 in order to effectuate the solution. That is, under the present scheme, DSP 110 is interrupted 160 times in every 20 msec interval (as measured by the PSTN clock) with respect to PCM input stream 104. In addition, DSP 110 is interrupted approximately 160 times in every 20 msec interval (as measured by the PSTN clock) with respect to PCM samples output on PCM output stream 122 (the interrupts are approximately 160 because, as has been discussed, the number of PCM samples actually placed in PCM sample output buffer depend upon the mismatch between the CDMA and PSTN clocks). Consequently, the present solutions to the foregoing identified problems results in approximately 320 interruptions of DSP 110 every 20 msec or 16,000 interruptions per second. From a computational standpoint, such a high number of interruptions is inefficient. That is, since DSP 110 is responsible for controlling all processing within CDMA voice coding subsystem 100, it is apparent that it would be advantageous to reduce the number of interrupts of DSP 110 necessary to achieve the solution to the foregoing problems.
In addition to the foregoing noted problems, there were additional motivations for the present invention. One such motivation is that while in traditional methods there is only one call being handled per DSP 110, there is an impetus in the marketplace to go to multi-call: more than one call being handled per DSP 110. As can be seen, if an attempt to go to multi-call is made, the foregoing noted problems multiply (e.g., there are now as many interruptions of DSP 110 per call as before, except that these interruptions will be multiplied by the number of calls being handled by DSP 110). Thus, marketplace pressure also indicates that it would be advantageous to find a way to maintain the efficacy of the prior art solution, yet do so in a way that reduces the number of interrupts per call.
It is therefore apparent that a need exists for a method and system which will provide a solution to the communication failure problems associated with lack of synchronization between CDMA system clocks and PSTN system clocks, but which will do so in a more computationally efficient way.
It is therefore one object of the present invention to provide a method and system to be utilized with wireless communications systems having cellular architectures which utilize digital clocked technologies (such as Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA) or similar spread spectrum technologies), and which interface with public switched telephone networks (PSTNs).
It is yet another object of the present invention to provide a method and system, to be utilized with wireless communications systems having cellular architectures which utilize digital clocked technologies (such as TDMA, CDMA or similar spread spectrum technologies), and which interface with PSTNs, wherein the method and system increase the reliability of such wireless communications systems by avoiding communication failures at the wireless communication system-PSTN system interfaces.
The method and system achieve their objects via communications equipment adapted to do the following: designate a first data-producing system controlled by a first clock; designate a second data-producing system controlled by a second clock; record a timing mismatch between the first clock and the second clock; and dynamically adjusting data flow between the first and the second system in response to the recorded timing mismatch. In one embodiment the first system is a CDMA system controlled by a GPS clock, and the second system is a PSTN system controlled by a PSTN clock.
The above, as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description.